Salt of weak bronsted base and bronsted acid as gelation retarder for crosslinkable polymer compositions

ABSTRACT

According to one embodiment, a treatment fluid for a well includes: (a) a water-soluble polymer, wherein the water-soluble polymer comprises a polymer of at least one non-acidic ethylenically unsaturated polar monomer; (b) an organic crosslinker comprising amine groups, wherein the organic crosslinker is capable of crosslinking the water-soluble polymer; (c) a salt of a weak Bronsted base and a Bronsted acid; and (d) water; wherein the treatment fluid is a crosslinkable polymer composition. According to another embodiment, a method for blocking the permeability of a portion of a subterranean formation penetrated by a wellbore is provided, the method including the steps of: (a) selecting the portion of the subterranean formation to be treated; (b) selecting estimated treatment conditions, wherein the estimated treatment conditions comprise temperature over a treatment time; (c) forming a treatment fluid that is a crosslinkable polymer composition comprising: (i) a water-soluble polymer, wherein the water-soluble polymer comprises a polymer of at least one non-acidic ethylenically unsaturated polar monomer; (ii) an organic crosslinker comprising amine groups, wherein the organic crosslinker is capable of crosslinking the water-soluble polymer; (iii) a salt of a weak Bronsted base and a Bronsted acid; and (iv) water; (d) selecting the water-soluble polymer, the crosslinker, the salt of a weak Bronsted base and a Bronsted acid, and the water, and the proportions thereof, such that the gelation time of the treatment fluid is at least 1 hour when tested under the estimated treatment conditions; and (e) injecting the treatment fluid through the wellbore into the portion of the subterranean formation.

BACKGROUND

1. Technical Field

The invention generally relates to producing crude oil or natural gasfrom a well drilled into a subterranean formation. More particularly,the invention is directed to improved treatment fluids and methods thatare capable of forming crosslinked gels in subterranean formations. Aparticular application of the invention is for conformance control.Production of unwanted water from a hydrocarbon producing well can be alimiting factor in the productive life of a well.

2. Background Art

Oil or gas is naturally occurring in certain subterranean formations. Asubterranean formation having sufficient porosity and permeability tostore and transmit fluids is referred to as a reservoir. A subterraneanformation that is a reservoir for oil or gas may be located under landor under a seabed offshore. Oil or gas reservoirs are typically locatedin the range of a few hundred feet (shallow reservoirs) to a few tens ofthousands of feet (ultra-deep reservoirs) below the ground or seabed.

In order to produce oil or gas, a wellbore is drilled into asubterranean formation that is an oil or gas reservoir. A wellbore caninclude an openhole or uncased portion. A wellbore can have vertical andhorizontal portions, and it can be straight, curved, or branched.

Various types of treatments are commonly performed on wells orsubterranean formations penetrated by wells. For example, stimulation isa type of treatment performed on a subterranean formation to restore orenhance the productivity of oil or gas from the subterranean formation.Stimulation treatments fall into two main groups: hydraulic fracturingand matrix treatments. Fracturing treatments are performed above thefracture pressure of a subterranean formation to create or extend afracture in the rock. The fracture is propped open with sand or otherproppant to provide a highly permeable flow path between the formationand the wellbore. Matrix treatments are performed below the fracturepressure of a subterranean formation. Matrix treatments can include, forexample, treatments to consolidate a matrix of unconsolidated rockparticles so that less particulate is produced with the producedhydrocarbon or to alter the permeability of the matrix of a subterraneanformation to improve fluid flow through the formation.

When oil or gas is produced from subterranean formations, water oftenaccompanies the produced oil or gas. The source of the water can be awater producing zone communicating with the oil or gas producingformation through a fracture, high-permeability streak,high-permeability zone, and the like, or it can be caused by a varietyof other occurrences which are well known to those skilled in the art,such as water coning, water cresting, bottom water, lateral channeling,channeling at the wellbore, etc.

In addition, the source of the water can be the result of waterfloodtechniques, which is a type of secondary recovery to improve productionof oil. Secondary recovery is the second stage of hydrocarbon productionduring which an external fluid such as water, gas, or alternating bothfluids is injected into the reservoir through one or more injectionwells penetrating a subterranean formation that has fluid communicationwith a production well. The purpose of secondary recovery is to maintainreservoir pressure and to displace hydrocarbons toward the wellbore of aproduction well. In waterflooding, water is injected into a reservoir todisplace residual oil. The water from injection wells sweeps thedisplaced oil toward a production well. Potential problems associatedwith waterflood techniques include inefficient recovery due to variablepermeability and other conditions affecting fluid transport within thereservoir. Early water breakthrough to the production well may causeproduction and surface processing problems.

Conformance control is a type of well treatment directed to improve theinjection or production profile of a well. Conformance control issometimes referred to as profile modification. Conformance controlencompasses procedures that enhance recovery efficiency, such as byreducing the proportion of water produced with the oil or gas. Problemsof high water production caused by permeability variations in asubterranean formation have been corrected, for example, by reducing thepermeability of a portion of the subterranean formation having highpermeability and low oil or gas content.

There are at least two types of methods for reducing the permeability ofa portion of a subterranean formation. One method involves the injectionof a polymer that is capable of being crosslinked to form a gel withinthe matrix of the subterranean formation. The gel physically blocksfluid flow through the portion of the formation in which the gel hasbeen placed, directing all fluid flow around the portion of theformation or inducing the production from the non-drained portions. Thismethod is sometimes referred to as permeability blocking. As a result ofthis kind of treatment, fluid flow is directed through other portions ofthe subterranean formation having lower permeability. The polymercompositions for use in this method are sometimes referred to ascrosslinkable polymer compositions.

Another method for reducing the permeability of a subterranean formationinvolves the injection of a chemical that attaches to adsorption siteson the rock surfaces within the matrix of the subterranean formation.The attached chemical is adapted to reduce the water permeabilitythrough the formation without substantially reducing the hydrocarbonpermeability. These chemicals are sometimes referred to as relativepermeability modifiers.

Crosslinkable polymer compositions have included, for example,water-soluble polymers including copolymers of acrylamide and acrylicacid crosslinked with chromium or other transition metal ions. Inaccordance with an early technique, an aqueous solution of one or moreof the polymers or copolymers mixed with a crosslinking metal ion isinjected into the subterranean formation and allowed to cross-linktherein. However, it has heretofore been found that the metalcross-linked gels formed have often been ineffective at hightemperatures, i.e., at temperatures above about 180° F. (82° C.) becauseof the instability of the crosslinker or polymer. This has resulted inuncontrolled crosslinking rates (too rapid), crosslinker precipitation,polymer degradation, or inefficient solution propagation through therock matrix. In attempts to correct these problems, the crosslinkingmetal ion has been coordinated with a ligand such as acetate orpropionate to slow the reaction of the metal ion with the polymer. Whilethis and other techniques have been utilized successfully, the use ofsome metal ions, e.g., chromium, has adverse environmental effects, andthe metal ion used can be adsorbed by formation materials whereby it isprevented from functioning to crosslink the polymer.

U.S. Pat. No. 4,773,481 to Allison et al. entitled “ReducingPermeability of Highly Permeable Zones in Underground Formations,”issued on Sep. 27, 1988, which is incorporated herein by reference inits entirety, describes a process for reducing the permeability of asubterranean formation by the cross-linking of water-soluble polymers ofpolyalkylene imines and polyalkylenepolyamines with certain polymerswhich are anionic or hydrolyzable to form anionic polymers. Examples ofthe anionic polymers are polyacrylamide and alkylpolyacrylamides,copolymers of polyacrylamide and alkylpolyacrylamides with ethylene,propylene and styrene, polymaleic anhydride and polymethylacrylate, andhydrolysis products thereof. As described in the patent, when thewater-soluble polymer and the anionic polymer are mixed, a viscous gelis quickly formed. In use, a solution of the water-soluble polymer ispumped into the subterranean formation first, followed by water todisplace the water-soluble polymer from the wellbore to thereby preventpremature gelling upon introduction of the anionic polymer. Thereafter,the anionic polymer is pumped into the formation. This three-stepprocedure has a number of disadvantages in practice and is costly toperform, but it is necessary because the water-soluble polyalkyleneimine or polyalkylenepolyamine reacts very quickly with the anionicpolymer and cannot be premixed without premature gelation.

U.S. Pat. No. 5,836,392 having named inventor Phillip LanceUrlwin-Smith, entitled “Oil And Gas Field Chemicals,” issued on Nov. 17,1998, and assigned of record to Halliburton Energy Services, Inc., whichis incorporated herein by reference in its entirety, discloses a methodfor conformance control of a reservoir comprising injecting into a zoneof the reservoir an aqueous solution of a co-polymer comprising at leastone ethylenically unsaturated polar monomer and at least onecopolymerizable ethylenically unsaturated ester formed from a hydroxycompound of the formula ROH wherein R is a selected alkyl group, alkenylgroup, cycloalkyl group, aryl group or such groups substituted with from1 to 3 hydroxyl, ether or thio ether groups or a heterocyclic orselected heterocyclic alkylene group and at least one heteroatomselected from oxygen, nitrogen and sulfur and a selected alkenoic oraralkenoic carboxylic acid or sulfonic or phosphoric acid together witha crosslinking agent comprising a multi-valent metal ion capable ofcrosslinking an acrylic acid polymer to form a viscous gel. The injectedfluid is flowed through at least a portion of a high permeability regionwithin said zone wherein it is heated to an elevated temperaturewhereupon crosslinking of the polymers occurs to form a substantiallynon-flowable gel within said high permeability region. The crosslinkingof the injected fluid to form the non-flowable gel within the formationreduces the permeability of said region in said zone.

U.S. Pat. No. 6,192,986 to Phillip Lance Urlwin-Smith, entitled“Blocking Composition For Use In Subterranean Formation,” issued on Feb.27, 2001, and assigned of record to Halliburton Energy Services, Inc.,which is incorporated herein by reference in its entirety, describes away of avoiding the use of metal ion cross-linking agents and ofcontrolling the gelling rate of polymers whereby premixes of polymer anda gelling agent can be made and safely injected into a downholeformation without serious risk of premature gelation. The compositioncomprises a water-soluble copolymer comprising (i) at least onenon-acidic ethylenically unsaturated polar monomer and (ii) at least onepolymerizable ethylenically unsaturated ester; and (iii) at least oneorganic gelling agent, characterized in that the gelling agent is apolyalkyleneimine, polyfunctional aliphatic amine, an aralkylamine, or aheteroaralkylamine. The gelling agents are free from metal ions, and arepreferably water-soluble polymers capable of cross-linking thecopolymers. Among the preferred water-soluble polymers for use asgelling agents are polyalkyleneimines, polyalkylenepolyamines, andmixtures thereof. Additional details concerning these polymers and theirpreparation are disclosed in U.S. Pat. No. 3,491,049, which is alsoincorporated herein by reference in its entirety. The preferredpolyalkylenepolyamines are the polymeric condensates of lower molecularweight polyalkylenepolyamines and a vicinal dihaloalkane. Thepolyalkyleneimines are best illustrated by polymerized ethyleneimines orpropyleneimine. The polyalkylenepolyamines are exemplified bypolyethylene and polypropylenepolyamines. Other gelling agents which canbe used include water-soluble polyfunctional aliphatic amines, aralkylamines, and heteroaralkylamines optionally containing other heteroatoms. The method of conformance control of a subterranean reservoircomprises: (a) injecting into a formation an aqueous solution of acomposition of the invention; (b) allowing the solution to flow throughat least one permeable zone in said formation; and (c) allowing thecomposition to gel. As the solution is pumped downhole and permeatesinto the zone, it heats up and eventually reaches the downholetemperature after which gelling occurs.

U.S. Pat. No. 6,176,315 to B. R. Reddy, Larry Eoff, Jiten Chatterji, SanT. Tran, and Dwyann Dalrymple, entitled “Preventing Flow ThroughSubterranean Zones,” issued on Jan. 23, 2001, and assigned of record toHalliburton Energy Services, Inc., which is incorporated herein byreference in its entirety, discloses methods of preventing the flow ofwater or gas or both through a subterranean zone having a hightemperature and a depth such that a long pumping time is required toplace a sealing composition therein. The methods basically comprise thesteps of preparing a polymeric sealing composition comprised of water, across-linking agent, and a selected water-soluble polymer, which reactswith the cross-linking agent and forms a sealing gel which is stable fora desired period of time at the temperature of the zone and has apumping time before gelation in the presence of the cross-linking agent,whereby the composition can be pumped to the depth of the zone andplaced therein. Thereafter, the sealing composition is pumped into thezone and allowed to form a sealing gel therein. A “gelation acceleratingagent” can be utilized to reduce pumping time before gelation at a giventemperature. The gelation accelerating agent can be a pH controlcompound such as an alkali metal carbonate, bicarbonate or hydroxide, amineral acid such as hydrochloric acid, an organic acid such as aceticacid, a Lewis acid such as boric acid or other compounds such asammonium chloride, urea and lactose. Of these, boric acid is preferred.When utilized, boric acid is added to the sealing compositions of thisinvention in a general amount in the range of from about 0.005% to about0.1% by weight of the composition.

U.S. Pat. No. 6,196,317 to Mary Anne Hardy, entitled “Method andComposition for Reducing the Permeabilities of Subterranean Zones,”issued on Mar. 6, 2001, and assigned of record to Halliburton EnergyServices, Inc., which is incorporated herein by reference in itsentirety, describes the steps of introducing an aqueous solution of achelated organic gelling agent and a copolymer of a non-acidicethylenically unsaturated polar monomer and an ethylenically unsaturatedester into a subterranean zone, and then allowing the aqueous solutionto form a cross-linked gel in the zone. The chelated organic gellingagent is comprised of a water-soluble polyalkylene imine chelated with ametal ion, preferably polyethylene imine chelated with zirconium. Thenon-acidic ethylenically unsaturated polar monomer in the copolymer isan amide of an unsaturated carboxylic acid, preferably acrylamide, andthe ethylenically unsaturated ester in the copolymer is formed of ahydroxyl compound and an ethylenically unsaturated carboxylic acid suchas acrylic acid, methacrylic acid and the like. A preferred unsaturatedester is t-butyl acrylate. In a further aspect, instead of utilizing theabove-described copolymer which is rapidly cross-linked by the chelatedgelling agent once the chelated gelling agent disassociates, thecopolymer can be stabilized whereby it does not cross-link as rapidly athigh temperatures and also has greater long-term gel strength afterbeing cross-linked by forming it into a terpolymer or a tetrapolymer.That is, instead of a copolymer, the above-described non-acidicethylenically unsaturated polar monomer, preferably acrylamide, and theethylenically unsaturated ester, preferably t-butyl acrylate, arereacted with AMPS® (2-acrylamido-2-methylpropane sulfonic acid) and/orN-vinylpyrrolidone to produce a terpolymer, e.g., polyacrylamide/t-butylacrylate/AMPS® or polyacrylamide/t-butyl acrylate/N-vinylpyrrolidone ora tetrapolymer, e.g., polyacrylamide/t-butylacrylate/AMPS®/N-vinylpyrrolidone. The most preferred terpolymer ispolyacrylamide/t-butyl acrylate/N-vinylpynolidone. The compositions forreducing the permeability of a subterranean zone are basically comprisedof water, a copolymer of an ethylenically unsaturated polar monomer, andan ethylenically unsaturated ester or a terpolymer or tetrapolymer ofthe aforesaid polar monomer and ester with AMPS® and/orN-vinylpyrrolidone, and a chelated organic gelling agent.

As an example of a relative permeability modifier, U.S. Pat. No.6,476,196 to Larry Eoff, Raghava Reddy, and Eldon Dalrypmple, entitled“Methods of Reducing Subterranean Formation Water Permeability,” issuedNov. 5, 2002, and assigned to Halliburton Energy Services, Inc., whichis incorporated herein by reference in its entirety, disclosesintroducing into the formation a water flow resisting chemical whichattaches to adsorption sites on surfaces within the porosity of theformation and reduces the water permeability thereof withoutsubstantially reducing the hydrocarbon permeability thereof. The waterflow resisting chemical is comprised of a polymer of at least onehydrophilic monomer and at least one hydrophobically modifiedhydrophilic monomer.

U.S. Pat. No. 6,838,417 to Ron C. M. Bouwmeester and Klass A. W. VanGijtenbeek, entitled “Compositions and Methods Including Formate Brinesfor Conformance Control,” issued Jan. 4, 2005, and assigned toHalliburton Energy Services, Inc., which is incorporated herein byreference in its entirety, discloses compositions and methods areprovided for reducing the permeability of subterranean zones. Moreparticularly, water-soluble polymeric compositions which formcrosslinked gels in the zones. In general, the composition comprises (a)at least one water-soluble polymer; (b) at least one organic gellingagent capable of cross-linking the water-soluble polymer; and (c) atleast one water-soluble formate. More preferably, the water-solublepolymer is a copolymer of (i) at least one non-acidic ethylenicallyunsaturated polar monomer, and (ii) at least one polymerizableethylenically unsaturated ester. The gelling agent is preferably apolyalkyleneimine, polyfunctional aliphatic amine, an aralkylamine, anda heteroaralkylamine. The preferred water-soluble formate is selectedfrom the group consisting of ammonium formate, lithium formate, sodiumformate, potassium formate, rubidium formate, cesium formate, andfrancium formate. Water is used to make an aqueous composition prior touse in a subterranean formation. The methods of this invention forreducing the permeability of a subterranean zone are comprised of thesteps of introducing an aqueous composition according to the inventioninto a subterranean zone, and then allowing the aqueous composition toform a cross-linked gel in the zone. Preferably, the method includes thestep of subsequently producing hydrocarbons from the subterraneanformation.

U.S. Pat. No. 7,091,160 to Bach Dao et al., entitled “Methods andCompositions for Reducing Subterranean Formation Permeabilities,” issuedAug. 15, 2006, and assigned to Halliburton Energy Services, Inc., whichis incorporated herein by reference in its entirety, discloses methodsand compositions for reducing the permeabilities of subterraneanformations or zones are provided. The methods are comprised ofintroducing an aqueous composition into the formation or zone comprisedof water, a water soluble organic polymer, an organic gelling agent forcross-linking the organic polymer and a gel retarder comprised of achemical compound (e.g., polysuccinimide or polyaspartic acid) thathydrolyzes or thermolyzes to produce one or more acids in thecomposition and then allowing the aqueous composition to form across-linked gel in the formation or zone.

U.S. Pat. No. 7,128,148 to Larry S. Eoff and Michael J. Szymanski,entitled “Well Treatment Fluid and Methods for Blocking Permeability ofa Subterranean Zone,” issued Oct. 31, 2006, and assigned to HalliburtonEnergy Services, Inc., which is incorporated herein by reference in itsentirety, discloses a well treatment fluid for use in a well, the welltreatment fluid comprising water, a water-soluble polymer comprising atleast one unit of vinyl amine, and an organic compound that iscrosslinked with the polymer. It also discloses a method of treating asubterranean formation penetrated by a wellbore, the method comprisingthe steps of: (a) forming a treatment fluid comprising water, awater-soluble polymer comprising at least one unit of vinyl amine, andan organic compound that is crosslinked with the polymer; and (b)introducing the treatment fluid through the wellbore and into contactwith the formation.

U.S. Pat. No. 7,287,587 to B. Raghava Reddy, Larry S. Eoff, Eldon D.Dairymple, and Julio Vasquez, entitled “Crosslinkable PolymerCompositions and Associated Methods,” issued Oct. 30, 2007, and assignedto Halliburton Energy Services, Inc., which is incorporated herein byreference in its entirety, discloses crosslinkable polymer compositionscomprising an aqueous fluid; a water-soluble polymer comprising carbonylgroups; an organic crosslinking agent capable of crosslinking thewater-soluble polymer comprising carbonyl groups; and a water-solublecarbonate retarder. Methods comprising: providing a crosslinkablepolymer composition; introducing the crosslinkable polymer compositioninto a portion of a subterranean formation; and allowing thecrosslinkable polymer composition to form a crosslinked gel in theportion of the subterranean formation.

Halliburton Energy Services, Inc. has employed a crosslinkable polymersystem of a copolymer of acrylamide and t-butyl acrylate, where thecrosslinking agent is polyethylene imine. These materials arecommercially available from Halliburton Energy Services, Inc. as part ofthe H₂Zero™ conformance control service. The H₂Zero™ service employs acombination of HZ-10™ polymer and HZ-20™ crosslinker. HZ10™ polymer is alow molecular weight polymer consisting of polyacrylamide and anacrylate ester. More particularly, HZ10™ polymer is a co-polymer ofacrylamide and t-butyl acrylate (“PAtBA”). The HZ20™ crosslinker is apolyethyleneimine (which is not chelated). The H₂Zero™ service forconformance control includes mixing the HZ-10™ polymer with the HZ20™crosslinker and injecting the fluid mixture into a well. The relativeamounts of HZ-10™ polymer and HZ20™ crosslinker to be used in thepreparation of H₂Zero™ can be adjusted to provide gelling within aspecified time frame (within certain limits) based on reactionconditions such as temperature and pH. For example, the amount of HZ20™crosslinker necessary for gelling is inversely proportional totemperature wherein higher amounts of HZ-20™ crosslinker are required atlower temperatures to effect formation of a viscous gel. Adjustment ofthe H₂Zero™ conformance control service to provide optimum gelling time(within certain limits) as a function of temperature and/or pH is knownto one of ordinary skill in the art.

More particularly, it is well known that the gelation time of the HZ10™polymer and HZ20™ crosslinker decreases with increasing temperature. Itis also believed that a pH of equal to or greater than 10 was helpful toincrease the gelation time.

Although the above-described water-soluble polymer systems crosslinkedwith organic crosslinkers are generally believed to be thermally stable,for example, it is believed the crosslinked gel of the H₂Zero™ serviceis stable up to about 400° F. (204° C.). However, the use of the polymergel system in conformance applications at matrix temperatures close tothe gel stability temperature is limited by the inadequately short pumptimes. When gelling compositions utilizing gelation retarders such asthe carbonate salts, as described in U.S. Pat. No. 7,287,587 discussedearlier, are used in field water, rich in divalent ions such as calciumion and magnesium which contribute to the hardness of water, or seawater divalent and multivalent ions, precipitation of solids, presumablycomposed of insoluble magnesium and calcium carbonates, and otherinsoluble salts, are formed upon mixing the components. Formation ofsuch solid precipitates renders injection of fluids into the porosity offormation matrix very difficult or impossible without using highinjection pressure with the possibility of such pressures exceeding thefracture pressure of the formation matrix. Thus, there are continuingneeds for improved compositions and methods for blocking thepermeabilities of subterranean formations or zones using a crosslinkablepolymer composition where the crosslinking of the polymer is effectivelyand simply controlled at high temperatures.

SUMMARY OF THE INVENTION

The present invention provides compositions and methods for use intreating a subterranean formation.

According to one embodiment, the invention provides a treatment fluidfor use in a subterranean formation, the treatment fluid comprising: (a)a water-soluble polymer, wherein the water-soluble polymer comprises apolymer of at least one non-acidic ethylenically unsaturated polarmonomer; (b) an organic crosslinker comprising amine groups, wherein theorganic crosslinker is capable of crosslinking the water-solublepolymer; (c) a salt of a weak Bronsted base and a Bronsted acid; and (d)water; wherein the treatment fluid is a crosslinkable polymercomposition.

According to another embodiment, the invention provides a method forblocking the permeability of a portion of a subterranean formationpenetrated by a wellbore, the method comprising the steps of: (a)selecting the portion of the subterranean formation to be treated; (b)selecting estimated treatment conditions, wherein the estimatedtreatment conditions comprise temperature over a treatment time; (c)forming a treatment fluid that is a crosslinkable polymer compositioncomprising: (i) a water-soluble polymer, wherein the water-solublepolymer comprises a polymer of at least one non-acidic ethylenicallyunsaturated polar monomer; (ii) an organic crosslinker comprising aminegroups, wherein the organic crosslinker is capable of crosslinking thewater-soluble polymer; (iii) a salt of a weak Bronsted base and aBronsted acid; and (iv) water; (d) selecting the water-soluble polymer,the crosslinker, the salt of a weak Bronsted base and a Bronsted acid,and the water, and the proportions thereof, such that the gelation timeof the treatment fluid is at least 1 hour when tested under theestimated treatment conditions; and (e) injecting the treatment fluidthrough the wellbore into the portion of the subterranean formation.

As used herein, the words “comprise,” “have,” “include,” and allgrammatical variations thereof are each intended to have an open,non-limiting meaning that does not exclude additional elements or steps.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, “subterranean formation” refers to the fundamental unitof lithostratigraphy. A subterranean formation is a body of rock that issufficiently distinctive and continuous that it can be mapped. In thecontext of formation evaluation, the term refers to the volume of rockseen by a measurement made through the wellbore, as in a log or a welltest. These measurements indicate the physical properties of thisvolume, such as the property of permeability. As used herein, a “zone”refers to an interval or unit of rock along a wellbore that isdifferentiated from surrounding rocks on the basis of hydrocarboncontent or other features, such as faults or fractures.

As used herein, a “well” includes a wellbore and the near-wellboreregion of rock surrounding the wellbore. As may be used herein, “into awell” means and includes into any portion of the well, including intothe wellbore of the well or into a near-wellbore region of asubterranean formation along a wellbore.

As used herein, the word “treatment” refers to a treatment for a well orsubterranean formation that is adapted to achieve a specific purpose,such as stimulation, isolation, or conformance control, however, theword “treatment” does not necessarily imply any particular purpose. Atreatment for a well or subterranean formation typically involvesintroducing a treatment fluid into a well.

As used herein, a “treatment fluid” refers to a fluid used in atreatment of a well or subterranean formation. A treatment fluid istypically adapted to be used to achieve a specific treatment purpose,such as stimulation, isolation, or conformance control, however, theword “treatment” in the term “treatment fluid” does not necessarilyimply any particular action by the fluid. As used herein, a “treatmentfluid” means the specific composition of a fluid at or before the timethe fluid is introduced into a wellbore.

As used herein, a “fluid” refers to an amorphous substance having acontinuous phase that tends to flow and to conform to the outline of itscontainer when tested at a temperature of 77° F. (25° C.) and a pressureof 1 atmosphere. A fluid can be homogeneous or heterogeneous. Ahomogeneous fluid consists of a single fluid phase with uniformproperties throughout. A heterogeneous fluid consists of at least onefluid phase and at least one other phase, which can be another fluid ora different phase, wherein the other phase has different properties.Examples of a homogeneous fluid include water, oil, or a solution of oneor more dissolved chemicals. An example of a heterogeneous fluid is adispersion. A dispersion is system in which one phase is dispersed inanother phase. An example of a dispersion is a suspension of solidparticles in a liquid phase. Another example of a dispersion is anemulsion. Further, a fluid can include an undissolved gas, whichundissolved gas can be used, for example, for foaming the fluid. Anaqueous fluid is a fluid that is either a homogeneous aqueous solutionor a heterogeneous fluid wherein the continuous phase is an aqueoussolution. An aqueous solution is a solution in which water is thesolvent.

Preferably, the treatment fluid according to the invention is acrosslinkable polymer composition. As used herein, a “crosslinkablepolymer composition” refers to a composition that under the appropriateconditions (e.g., mixing, time, and temperature) forms a crosslinkedgel. As used herein, a “crosslinked gel” refers to a semi-rigid,jelly-like mass formed when a polymer and crosslinking agent combinethrough a crosslinking reaction.

After placing in a portion of a subterranean formation under sufficientconditions for crosslinking, the crosslinkable polymer composition isexpected to produce a crosslinked gel therein, which can at leastpartially block the flow of water and other fluid through the portion ofthe subterranean formation. The crosslinkable polymer composition tendsto flow into any fractures and high permeability streaks in thesubterranean formation. After gelling in such portions of thesubterranean formation, the crosslinked gel at least partially blocksfluid flow and directs fluid flow around such fractures or highpermeability streaks in the formation and instead through lowerpermeability portions of the formation. The basic function of thecrosslinked gel is to physically fill and block the permeability of aportion of a subterranean formation.

This blocking action of a crosslinked gel is in contrast to the actionof a relative permeability modifier, which is a chemical that attachesto adsorption sites on surfaces within the porosity of a subterraneanformation and reduces the water permeability thereof withoutsubstantially reducing the hydrocarbon permeability thereof. The primaryfunctionality of a relative permeability modifier is to modify thepolarity characteristics of the surfaces of the rock within theformation, which tends to favor the relative flow of either water or oilthrough the formation.

The present invention can be particularly directed to crosslinkablepolymer compositions and associated methods that form a crosslinked gelfor physically blocking the permeability of a portion of a subterraneanformation.

It is important, however, that a crosslinkable polymer composition doesnot begin to build viscosity before it is placed into the desiredportion of a subterranean formation. If it builds viscosity too quickly,this would interfere with pumping and placement of the crosslinkablepolymer composition into the formation.

As used herein, the “gelation time” refers to the time a crosslinkablepolymer composition under particular conditions takes to begin buildingviscosity. The gelation time can vary widely depending on a number offactors, including, for example, the nature of the crosslinkablecomposition and the nature of conditions the crosslinkable polymercomposition is subjected to. The nature of the crosslinkable compositionincludes, for example, the nature of the polymer, the nature of thecrosslinking agent, the nature of any catalyst, the nature of the fluid,the concentrations of the various components in the fluid, and the pH ifthe composition is an aqueous solution. The nature of the conditionsinclude, for example, any shear conditions, pressure conditions, and thetemperature conditions from the time of forming the crosslinkablepolymer composition to at least the time of placement in a subterraneanformation. Regarding temperature conditions, the general rule, ofcourse, is that the higher the temperature, the faster the rate of achemical reaction, including, for example, a crosslinking reaction.Therefore, the higher the temperature conditions, the shorter thegelation time for a particular crosslinkable polymer composition underotherwise identical conditions.

Gelation time can be determined, for example, with a dynamic coaxialcylinder, controlled shear rate rheometer that allows viscositymeasurements under pressure at elevated temperatures over time. Anexample of such a rheometer is a High-Pressure PVS Rheometer(commercially available from Brookfield Engineering Laboratories Inc.,Middleboro, Mass.). Plotting such measurements of viscosity versus time,the gelation time is determined at the inflection point of the curve. Adescription of gel time measurement methods is given in U.S. Pat. No.6,176,315, which is incorporated herein by reference.

The desired gelation time for a crosslinkable polymer composition variesdepending on the specific treatment application in a specific well. Forexample, for treating wells of considerable depth, a longer gelationtime may be required to allow the crosslinkable composition to be pumpedto a desired location in a subterranean formation before the compositionforms a crosslinked gel. In addition, a wide range of temperatureconditions can be encountered in particular applications, which presentchallenges to the use of crosslinkable polymer compositions andassociated methods. For example, if the bottomhole temperature of thesubterranean formation is sufficiently high, the crosslinkable polymercomposition gelation time may be too short to allow time for properplacement of the composition. As used herein, the bottomhole temperature(“BHT”) is the downhole temperature measured or calculated at a point ofinterest, such as a portion of a subterranean formation to be treated.The BHT, without reference to circulating or static conditions, istypically associated with producing conditions. The gelation time of aparticular crosslinkable polymer composition can be effected by otherconditions to which it is subjected, such as pressure and shear rateduring pumping and placement.

According to the invention, the composition of a crosslinkable polymercomposition is adapted such that the gelation time under the estimatedtreatment conditions over a treatment time is not too short for adesired treatment purpose. As used herein, the estimated treatmentconditions include at least an estimated temperature profile for thetreatment fluid over the treatment time. The estimated treatmentconditions can additionally include an estimated shear rate andestimated pressure profile over the course of the treatment time. Itshould be understood that the any of the estimated temperature, shearrate, and pressure profiles over the treatment time can be constant,ramped, or otherwise varied over the treatment time. As used herein, a“treatment time” is the time under the treatment conditions measuredfrom the time of formation of the crosslinkable polymer compositionthrough the time the crosslinkable polymer composition becomes acrosslinked gel. The gelation time under the estimated treatmentcondition should be at least sufficient for desired placement of thecrosslinkable polymer composition into a subterranean formation beforethe gelation time, whereby the crosslinkable polymer composition can beexpected to be placed as desired before it becomes a crosslinked gel.

For example, in a conformance control treatment using a treatment fluidcomprising a crosslinkable polymer composition, the treatment fluid ispumped down a wellbore and into the matrix of a subterranean formation.The amount of the treatment fluid to be pumped depends upon severalfactors, including the length of the formation to be treated along thewellbore and the desired depth of penetration outward from the wellbore.This depth of penetration may vary, but is typically at least 2 feetaway from the wellbore and may be as much as 25 feet away from thewellbore. It is typically desired to place the entire amount of thetreatment fluid into the formation of interest before the crosslinkablepolymer composition begins to build viscosity. Therefore, there is afinite amount of pumping time to place the treatment fluid.

One factor involved in determining this pump time is the depth of thezone of interest of a subterranean formation to be treated. In addition,injectivity tests can be performed on the zone of interest, typicallyusing brine solutions, which can indicate the rate at which fluids canbe pumped into the formation. Therefore, the amount of time required topump the treatment fluid into place in a subterranean formation can bedetermined.

In addition to the pump time, the estimated treatment conditions for atreatment can be determined by a person of skill in the art, includingbased on the depth, bottomhole temperature, and injectivity profile ofthe subterranean formation. As mentioned above, the estimated treatmentconditions include at least an estimated temperature profile for thetreatment fluid over the course of the treatment time. The estimatedtreatment conditions can additionally include an estimated shear rateprofile for the injection of the treatment fluid over the course of thetreatment time and an estimated pressure profile for the injection ofthe treatment fluid over the course of the treatment time. As a safetyfactor, the estimated treatment conditions are usually estimated to bemore extreme than the actual injection treatment conditions. Forexample, instead of estimating a temperature profile of increasingtemperature for the treatment fluid over the course of the treatmenttime, the estimated treatment conditions can assume that the temperatureis constant at the bottomhole temperature of the formation. Similarly,the shear rate may actually be zero after placement of the treatmentfluid in the formation, however, the estimated treatment conditions mayassume a constant shear rate. These will provide a margin againstpremature gelation of treatment using a crosslinkable polymercomposition.

According to current technology, the pump time for such a treatmentfluid is rarely determined to be less than about 1 hour. Accordingly,the required gelation time in accordance with the estimated treatmentconditions is usually determined to be at least 1 hour. In addition, atleast 1 hour is preferably added to the required gelation time as asafety factor against interruption or other difficulty during pumping,for example, in case the pumping operation is interrupted due to pumpbreakdown or other mechanical failures. Therefore, it is often desirableto provide a gelation time under the estimated treatment conditions thatis at least 2 hours. On the other hand, it is desirable to provide agelation time that is not too long, either. Accordingly, the gelationtime should be less than 100 hours under the estimated treatmentconditions. A preferred gelation time under the estimated treatmentconditions for a well treatment on a subterranean formation is usuallyin the range of about 2 hours to about 4 hours.

To help increase the gelation time of a crosslinkable polymercomposition under the applicable conditions, a pre-cool step can beemployed, which involved injecting a cooled fluid into the wellbore tolower the temperature profile of the wellbore and formation just priorto introducing a treatment fluid comprising a crosslinkable polymercomposition. In an embodiment of the method of the invention, it willsometimes be possible to reduce the volume of any pre-cool stage andconsequently the time and expense required to conduct a pre-cool step.In any case, as the treatment fluid is pumped downhole and permeatesinto a subterranean formation, it is heated up by the higher temperatureof the formation and eventually reaches equilibrium with the naturaldownhole temperature of the formation.

According to the methods of the present invention, the permeability ofthe portion of the subterranean formation to be treated is preferablyhigh, but the methods can be useful even if the permeability is as lowas about 1 mD.

1. TREATMENT FLUIDS

As mentioned, according to one embodiment, the invention provides atreatment fluid for use in a subterranean formation, the treatment fluidcomprising: (a) a water-soluble polymer, wherein the water-solublepolymer comprises a polymer of at least one non-acidic ethylenicallyunsaturated polar monomer; (b) an organic crosslinker comprising aminegroups, wherein the organic crosslinker is capable of crosslinking thewater-soluble polymer; (c) a salt of a weak Bronsted base and a Bronstedacid; and (d) water; wherein the treatment fluid is a crosslinkablepolymer composition.

Unless otherwise specified, any doubt regarding whether units are inU.S. or Imperial units, in the few cases where there is any difference,U.S. units are intended herein. For example, “gal/Mgal” means U.S.gallons per thousand U.S. gallons. In addition, unless otherwisespecified, any percentage means by weight.

A. Water-Soluble Polymer

A water-soluble polymer useful in the compositions of this invention isformed from at least one non-acidic ethylenically unsaturated polarmonomer. More preferably, the polymer is a copolymer of at least onenon-acidic ethylenically unsaturated polar monomer and at least oneethylenically unsaturated ester.

(i) Non-Acidic Ethylenically Unsaturated Polar Monomer

The non-acidic ethylenically unsaturated polar monomer may be derivedfrom an unsaturated carboxylic acid wherein the unsaturated group isvinyl or alpha methyl vinyl. The polar monomer formed from the acid isnon-acidic and is preferably a primary, secondary, or tertiary amide ofthe unsaturated carboxylic acid. The amide can be derived from ammoniaor a primary or secondary alkylamine, e.g., an alkyl amine having from 1to 10 carbon atoms which may also be substituted by at least onehydroxyl group. That is, the amide of the acid can be an alkylol amidesuch as ethanolamide. Examples of suitable non-acidic ethylenicallyunsaturated polar monomers are acryl amide, methacrylamide, and acrylicethanol amide. The non-acidic ethylenically unsaturated polar monomermay also be a vinyl heterocyclic compound with at least an oxygen,sulfur, or nitrogen atom in a ring with 3 to 8 carbon atoms, such as onewith at least one carbonyl group in the ring, e.g., N-vinyl pyrrolidone,N-vinyl caprolactam, or a vinyl pyridine.

(ii) Copolymer with Ethylenically Unsaturated Ester

The presence of the ester moiety in polymers for use in the invention isexpected to be unnecessary since the gelation retarder delays thegelation time and thus enables the copolymer to be premixed withcrosslinker before being pumped downhole. Nevertheless, the ester moietycan provide additional control of the gelation time and may be helpful.If the ester moiety is included in the copolymer, it is preferred thatthe ester group be such as to provide steric hindrance and, for thispurpose, bulky ester groups such as t-butyl, for example, are preferred.The precise delay in cross-linking and gelation caused by the estergroup will vary from copolymer to copolymer, as will be clear to thoseskilled in the art. Some experimental trial may, therefore, be necessaryto determine the optimum with any particular copolymer. The nature andamount of the ester will be such as to provide a delay in the gelationtime (compared to a homopolymer omitting any ester component),sufficient, for example, to enable a premix to be pumped into aformation without premature gelling.

The ethylenically unsaturated esters which can be used with thenon-acidic ethylenically unsaturated polar monomer described above toform a copolymer can be formed from an ethylenically unsaturatedcarboxylic acid and a hydroxyl compound. The ethylenically unsaturatedgroup is preferably in the alpha to beta or the beta to gamma positionrelative to the carboxyl group or may be further distant.

Preferred ethylenically unsaturated carboxylic acids for use in formingthe ethylenically unsaturated esters have in the range of from 3 to 20carbon atoms. Examples of these acids are acrylic acid, methacrylicacid, crotonic acid, and cinnamic acids.

The hydroxyl compound for use in forming the ethylenically unsaturatedesters is preferably an alcohol of the formula ROH, where R is ahydrocarbyl group. Preferred hydrocarbyl groups are alkyl groups havingfrom 1 to 30 carbon atoms, alkenyl groups having from 2 to 20 carbonatoms, cycloalkyl groups having from 5 to 8 carbon atoms, aryl groupssuch as aromatic hydrocarbyl groups having from 6 to 20 carbon atoms,and arylalkyl groups having from 7 to 24 carbon atoms. Specific examplesof R groups are methyl, ethyl, propyl, butyl, amyl, hexyl, octyl,2-ethylhexyl and decyl (including all stereoisomers), allyl, cyclohexyl,palmityl, stearyl, phenyl, and benzyl.

The R group of the hydroxyl compound may also be a hydrocarbyl groupsubstituted by at least one, e.g., from 1 to 3 substituents, such ashydroxyl, ether, and thioether groups. Electron donating groupsubstituents are preferred. Ether substituents are also preferred,especially alkoxy, aryloxy, and arylalkoxy in which the alkyl, aryl, andarylalkyl groups may be as described above. Preferably, the substituentis on the same carbon atom of the R group as is bonded to the hydroxylgroup in the hydroxyl compound with alkoxymethyl and arylalkyloxy methylgroups being preferred.

The R group of the hydroxyl compound may also comprise a heterocyclicgroup either for bonding directly to the hydroxyl group of ROH orseparated therefrom by an alkylene group having 1 to 4 carbon atoms suchas methylene. Thus, the R group may be a saturated or unsaturatedheterocyclic or heterocyclic alkylene group, e.g., having 3 to 8 carbonatoms and at least one or two ring heteroatoms selected from oxygen,nitrogen, and sulfur. Examples of such groups are furyl,tetrahydrofuryl, furfuryl and tetrahydrofurfuryl, pyranyl, andtetrahydropyranyl.

The hydroxyl compound may be a primary, secondary, iso, or tertiarycompound, preferably with a tertiary carbon atom bonded to the hydroxylgroup, e.g., tert-butyl and trityl. Preferred R groups are tert-butyl,trityl, methoxymethyl, benzyloxymethyl, and tetrahydropyranyl. Otherless preferred R groups include stearyl, isopropyl, ethyl, and methyl.The most preferred ester is t-butyl ester.

The ester is preferably substantially neutral as a fully esterifiedderivative of an acid, i.e., complete ester, rather than a partial esterwith free acid groups.

The copolymer can contain from about 0.01 to 50%, e.g. 0.1 to 40% or 1to 30%, especially 5 to 15% (by mole) of structural units from saidester(s) and 99.99 to 50% e.g. 99.9 to 60% or 99 to 70% or 95 to 85% (bymole) of structural units from said polar monomer(s) (please see U.S.Pat. No. 6,192,986, which is incorporated herein by reference in itsentirety). More preferably, the polar monomer is present in thecopolymer in an amount of about 85 to about 95 mole percent with theester monomer being present in an amount of from about 5 to about 15mole percent. The copolymer may be a block or non-block copolymer, aregular or random copolymer, or a graft copolymer whereby the esterunits are grafted onto a polymerized polar monomer, e.g., the estergrafted onto polyacrylamide.

In the more preferred compositions of the invention, the copolymer isformed from at least one polar monomer, preferably from 1 to 3 monomers,and at least one, preferably from 1 to 3, esters, and comprisesstructural units derived from said monomer(s) and ester(s). Mostpreferably, the copolymer consists essentially of said structural units.

The copolymer can be produced by conventional methods for copolymerizingethylenically unsaturated monomers in solution, emulsion, or suspension.

(iii) Other Monomers

In order to slow down the cross-linking of the crosslinkable polymercomposition and increase its gel strength after it is cross-linked, acopolymer, terpolymer or tetrapolymer formed from the above-describedpolar monomer with other monomers such as 2-acrylamido-2methylpropanesulfonic (AMPS®) acid and its salts, alkali and alkaline earth metalsalts of acrylic acid, alkacrylic acids (for example, methacrylic acid),styrene sulfonic acid, and/or N-vinylpyrrolidone in addition to or inplace of the above described ester can be substituted for or combinedwith the above-described copolymer. The terpolymer can contain fromabout 50 to about 98.9 mole percent of the polar monomer, from about0.01 to about 50 mole percent of the ester, and from about 1 to about 40mole percent of the AMPS® or N-vinylpyrrolidone monomer. Thetetrapolymer can contain from about 50 to about 97.9 mole percent of thepolar monomer, from about 0.01 to about 50 mole percent of the ester,from about 1 to about 20 mole percent of AMPS®, and from about 1 toabout 20 mole percent of N-vinylpyrrolidone. The terpolymer ortetrapolymer can be a block or non-block polymer, a regular or randompolymer, or a graft polymer. In addition, the solubility, molecularweight, viscosity, production, and other properties of the terpolymer ortetrapolymer should generally be as described above for the copolymer.Examples of such polymers are provided in U.S. Pat. No. 6,176,315, whichis incorporated by reference in its entirety.

(iv) Water Solubility of Polymer

The water-soluble polymer is soluble in water to the extent of at least10 grams per liter in deionized water at 25° C. More preferably, thewater-soluble polymer is also soluble to the extent of at least 10 gramsper liter in an aqueous sodium chloride solution of 32 grams sodiumchloride per liter of deionized water at 25° C. If desired, thewater-soluble polymer can be mixed with a surfactant to facilitate itssolubility in the water or salt solution utilized. The water-solublepolymer can have an average molecular weight in the range of from about50,000 to 20,000,000, most preferably from about 100,000 to about500,000. A water-soluble polymer having an average molecular weight ofabout 50,000 has a viscosity when dissolved in distilled water in theamount of about 3.6% by weight of the solution at 19° C. of from about10 to about 500 centipoise. Preferably, the polymer is shear thinnable,whereby the viscosity reduces by at least 10% on increasing shear rateby 10%.

B. Organic Crosslinker

As used herein, a “crosslinker” is a chemical that reacts with thewater-soluble polymer to chemically link by covalent bonds the polymermolecules, which helps increase the viscosity of the polymer insolution. As used herein, “organic crosslinker” means that thecrosslinker forms covalent bonds between water-soluble polymer and thecrosslinker, not ionic bonds. According to the invention, the organiccrosslinker for the water-soluble polymer is an organic compoundcomprising amine groups.

The crosslinker is water soluble in water to the extent of at least 10grams per liter in deionized water at 25° C. More preferably, thecrosslinker is also soluble to the extent of at least 10 grams per literin an aqueous sodium chloride solution of 32 grams sodium chloride perliter deionized water at 25° C.

Preferably, the crosslinker comprising amine groups is a polymer. Morepreferably, the organic crosslinker suitable for use in accordance withthis invention is selected from the group consisting of apolyalkyleneimine, polyfunctional aliphatic amine, an aralkylamine, aheteroaralkylamine, polyvinylamine and poly(vinylamine-co-vinylalcohol).Additional details concerning these polymers and their preparation aredisclosed in U.S. Pat. No. 3,491,049 and U.S. Pat. No. 7,128,148, thespecifications of which are incorporated herein by reference in itsentirety. The preferred polyalkylenepolyamines are the polymericcondensates of lower molecular weight polyalkylenepolyamines and avicinal dihaloalkane. The polyalkyleneimines are best illustrated bypolymerized ethyleneimines or propyleneimine. The polyalkylenepolyaminesare exemplified by polyethylene and polypropylenepolyamines. Otherorganic crosslinkers which can be used include water-solublepolyfunctional aliphatic amines, aralkyl amines, and heteroaralkylaminesoptionally containing other hetero atoms. Of these, polyethylene imineis most preferred.

Although less preferred, other organic crosslinkers that are expected tobe suitable for use in accordance with this invention are metal-ionchelated water-soluble polymers capable of cross-linking thewater-soluble polymer. The organic crosslinkers may be chelated asdescribed in U.S. Pat. No. 6,196,317, the specification of which isincorporated herein by reference in its entirety. Particularly suitablesuch water-soluble polymeric crosslinkers are chelated polyethyleneimines and polypropylene imines. Of these, chelated polyethylene imineis the most preferred. As mentioned, by chelating with a metal ion, thecrosslinker is prevented from cross-linking the copolymer prematurely athigh temperatures. That is, the polyalkylene imine utilized is chelatedwith a metal ion selected from the group consisting of zirconium ion,cobalt ion, nickel ion, ferric ion, titanium IV ion, and copper ion. Ofthese, zirconium ion is the most preferred.

C. Salt of Weak Bronsted Base and Bronsted Acid as Gelation Retarder

As used herein, “salt of a weak Bronsted base and a Bronsted acid”refers to the salt product of an acid-base reaction between a weakBronsted base and a Bronsted acid. The salts of weak Bronsted bases andBronsted acids include those formed from neutralization reactionsbetween Bronsted bases with either strong Bronsted acids or weakBronsted acids. A Bronsted acid is an acid which functions as an acid bydonating a proton to an acceptor molecule (called a Bronsted base), anda Bronsted base is a base which accepts a proton from a proton donormolecule (called a Bronsted acid). A salt of a weak Lewis base and aLewis acid is not preferred.

As used herein, a weak Bronsted base is defined as a base having apK_(b) equal to or greater than 3. The pK_(b) is defined as negativelogarithmic value of basicity constant (K_(b)) i.e., −log K_(b), for theequilibrium reaction when the weak Bronsted base is dissolved in wateras shown below in Equation 1:

$\begin{matrix}{{B + {H_{2}O}}\underset{K_{a}}{\overset{K_{b}}{\rightleftarrows}}\mspace{14mu}{{BH}^{+} + {OH}^{-}}} & {{Equation}\mspace{14mu} 1}\end{matrix}$where B is a base, and BH⁺ is referred to as the conjugate acid of thebase, B, and K_(a) is the acid dissociation constant of the conjugateacid, BH⁺ for the reverse reaction. The product of K_(a) and K_(b) is1.0×10⁻¹⁴. The larger the pK_(b) value, the weaker the base. The pK_(a)is defined as negative logarithm of K_(a) (−log K_(a)) of the conjugateacid form of the weak base. For a weak Bronsted base, the pKa is lessthan 11. Strong bases such as alkali and alkaline earth metalhydroxides, such as sodium hydroxide and magnesium hydroxide dissolve inwater by 100% ionization and the dissolution reaction is not anequilibrium reaction as in the case of weak bases. Such bases are notincluded in this invention. Weak Bronsted bases useful in the presentinvention include ammonia, primary, secondary and tertiary acyclic andcyclic aliphatic amines, aromatic amines such as aniline, toluidine,N,N-dimethylaniline, pyridine, and pyrrole, alkanolamines such asethanolamine, diethanolamine, triethanolamine and triisopropanolamine,diamines such as ethylenediamine andN,N,N′,N′-tetramethylethylenediamine, and triamines such asdiethyltriamine and the like.

As used herein, a Bronsted acid is defined as a proton donating acidhaving a pKa equal to or greater (i.e., higher positive value) than −10.Herein, pKa is defined as the negative logarithm (−log Ka) for thereaction between water and the Bronsted acid, as shown in Equation 2:

HA + H 2 ⁢ O ⁢ ⁢ K a K b ⁢ ⁢ A - + H 3 ⁢ O + Equation ⁢ ⁢ 2wherein HA is the Bronsted acid, and A⁻ is the conjugate base of theacid, HA. The Bronsted acid may be a strong acid, in which case theequilibrium reaction shown in Equation 2 favors completely the forwardreaction from left to right. Examples of such acids are mineral acidswhich include hydrochloric, sulfuric, phosphoric, and perchloric acidand the like but not carbonic acid; and organic acids such astrifluoroacetic acid, benzene sulfonic acids and methylsulfonic acids.Such strong acids characteristically have pK_(a) values less than 1. Forweak acids, both the forward and reverse reactions shown in theequilibrium Equation 2 contribute significantly, and pK_(a) values forsuch weak Bronsted acids are greater than 1. Examples of such acidsinclude carboxylic acids such as acetic, citric, benzoic and tartaricacids, phenols, dialkylmalonates, and acetoacetic esters, and the like.

The pH of such salts when dissolved in water as, for example 1%solutions, may be either in acidic, neutral or basic pH range, dependingon the relative magnitudes of pK_(a) and pK_(b) values of the Bronstedacids and Bronsted bases used in forming the salts. Thus, for example,

-   -   If pK_(a)<pK_(b), pH of the solution will be less than 7 and        consequently the solution will be acidic.    -   If pK_(a)>pK_(b), then pH of the solution will be more than 7        and hence the solution will be alkaline.    -   If pK_(a)=pK_(a), pH of the solution will be equal to 7 and        hence the solution will be neutral.

Suitable examples of salts of weak Bronsted bases and Bronsted acidsinclude, but are not limited to, ammonium tartarate, ammonium citrate,ammonium acetate, ammonium sulfate, ammonium dihydrogen phosphate,ammonium monohydrogen phosphate, ammonium phosphate, triethanolaminehydrochloride, anilinium hydrochloride, trisopropanolaminehydrochloride, and pyridinium hydrochloride. As described below, testswith these salts, for example with ammonium tartarate, ammonium sulfateand triethanolamine hydrochloride have shown their ability to delay thegelation time of the crosslinkable polymer composition (for example,H₂Zero™) comprising water-soluble polymers with amine-containing organiccrosslinkers, wherein the water-soluble polymers comprise a polymer ofat least one non-acidic ethylenically unsaturated polar monomer.

It is expected that this ability of a salt of a weak Bronsted base witha Bronsted acid to delay the cross-linking, can be extended to otherpolymer compositions that utilize amine-containing organic crosslinkers.Since salts of strong Bronsted bases and Bronsted acids are not includedin this invention, all salts of alkali metals such as sodium, potassium,cesium and alkaline earth metals such as calcium, magnesium and bariumdo not constitute part of this invention. In an embodiment, the salt ofa weak Bronsted base and a Bronsted acid is not ammonium halide orammonium formate. Also excluded are all carbonate salts. The formate andcarbonate salts are excluded from this invention because of theirtendency to form precipitates when added to hard water or any fieldwater containing divalent or trivalent metal ions.

It is expected that a salt of a weak Bronsted base and a Bronsted acidused according to the invention can be a single such salt, or it can beany combination of two or more salts of a weak Bronsted base and aBronsted acid.

As used herein, a “gelation retarder” is a chemical that when in asufficient concentration delays the gelation time of a crosslinkablepolymer composition relative to a similar composition without such ahigh concentration of the chemical. A gelation retarder in suchconcentration does not prevent the formation of a crosslinked gel. It isbelieved that a salt of a weak Bronsted base and a Bronsted acidfunctions as a gelation retarder when present in the composition at muchhigher concentrations than it would otherwise be naturally occurring inthe water or if added to such a composition for other purposes. Forexample, the salt of a weak Bronsted base and an acid should be presentin a higher concentration than would be used for catalytic purposes. Asused herein, a catalytic concentration is defined as less than 10 mole %based on the amine groups of the crosslinker.

The salt of a weak Bronsted base and a Bronsted acid as gelationretarder is present in at least an effective concentration in thecrosslinkable polymer composition such that the gelation time is atleast 1 hour when tested under the estimated treatment conditions for atreatment of a subterranean formation. More preferably, an otherwisesimilar treatment fluid except without the effective concentration ofthe gelation retarder would not have the desired gelation time of atleast 1 hour under the same estimated treatment conditions. A preferredgelation time under the estimated treatment conditions for a welltreatment on a subterranean formation is usually in the range of about 2hours to about 4 hours. Accordingly, most preferably, an otherwisesimilar treatment fluid except without the effective concentration ofthe gelation retarder would not have the desired gelation time of atleast 2 hours under the same estimated treatment conditions.

Preferably, the salt of a weak Bronsted base and a Bronsted acid iswater soluble. It is believed that to be effective as a gelationretarder, the salt of a weak base and an acid would be required in aconcentration of at least about 10 lb/Mgal (about 0.1% by weight) ofwater. Preferably, the salt of a weak Bronsted base and a Bronsted acidis present in a concentration of at least 25 lb/Mgal (about 0.3% byweight) of water, however, the concentration of the salt of a weakBronsted base and a Bronsted acid in the water of the treatment fluidpreferably does not exceed its solubility in the water at BHT.

Preferably, a salt of a weak Bronsted base and a Bronsted acid isselected that is be generally considered to be biodegradable and not along-term environmental pollutant.

D. Water

For downhole use in a well, the treatment fluid of the inventioncontains water in which the water-soluble polymer, the crosslinker, andthe salt of a weak Bronsted base and a Bronsted acid are dissolved. Anyconvenient source of water can be used, so long as it does not containcomponents that would adversely effect the compositions of theinvention, such as by causing precipitation. For example, the water foruse in the treatment fluid can be fresh water, seawater, natural brine,formulated brine, 2% KCl solution, and any mixture thereof. Formulatedbrine is manufactured by dissolving one or more soluble salts in water,natural brine, or seawater. Representative soluble salts are thechloride, bromide, acetate and formate salts of potassium, sodium,calcium, magnesium and zinc.

Preferably, the treatment fluid is made up just before use by mixing atleast the polymer, the crosslinker, the salt of a weak Bronsted base anda Bronsted acid, and the water, and then injecting the treatment fluidinto the formation.

E. Other Additives

The well treatment fluid of this invention generally will containmaterials well known in the art to provide various characteristics ofproperties to the fluid. Thus, the well treatment fluid can contain oneor more viscosifiers or suspending agents in addition to thewater-soluble polymer, surfactants, oxygen scavengers, alcohols, scaleinhibitors, corrosion inhibitors, weighting agents, soluble salts,biocides, fungicides, fluid loss control additives such as silica flour,seepage loss control additives, bridging agents, deflocculants,lubricity additives, shale control additives, pH control additives, andother additives as desired.

F. Preferred Treatment Fluids

More preferred compositions of this invention are comprised ofcombinations of the more preferred examples of a water-soluble polymer,an organic crosslinker, a salt of a weak Bronsted base and a Bronstedacid, and water.

For example, in the more preferred compositions, (a) the water-solublepolymer is preferably a copolymer of: (i) at least one non-acidicethylenically unsaturated polar monomer, and (ii) at least onepolymerizable ethylenically unsaturated ester. More preferably still,the non-acidic ethylenically unsaturated polar monomer in the polymer ispreferably an amide of an ethylenically unsaturated carboxylic acid,most preferably acrylamide. The ethylenically unsaturated ester in thecopolymer is preferably formed of a hydroxyl compound and anethylenically unsaturated carboxylic acid selected from the group ofacrylic acid, methacrylic acid, crotonic acid, and cinnamic acid. Thehydroxyl compound is preferably an alcohol having the formula ROHwherein R is a group selected from alkyl, alkenyl, cycloalkyl, aryl,arylalkyl, or an aromatic or heterocyclic group substituted with one ormore groups selected from hydroxyl, ether, and thioether groups. Mostpreferably, the ethylenically unsaturated ester monomer is t-butylacrylate. Most preferably, the water-soluble polymer ispoly(acrylamide/t-butyl acrylate).

Preferably, the organic crosslinker comprising amine groups is selectedfrom the group consisting of a polyalkyleneimine, polyfunctionalaliphatic amine, an aralkylamine, and a heteroaralkylamine. Mostpreferably, the organic crosslinker is polyethylene imine. Preferably,the treatment fluid does not include a crosslinker that forms ionicbonds with the water-soluble polymer.

The concentration of water-soluble polymer in the aqueous composition ispreferably from 500 to 100,000 ppm, in particular 500 to 10,000 ppm forpolymers of molecular weight of at least 1 million, and from 10,000 to100,000 ppm for polymers of molecular weight 50,000 to 1 million.Preferably, the concentration of the crosslinker in the aqueouscomposition is from 10 to 50,000 ppm, especially 10 to 1,000 ppm and1,000 to 50,000 ppm, respectively, for the high and low molecular weightcopolymers.

The presently preferred compositions of this invention are comprised ofa copolymer of acrylamide and t-butyl acrylate present in an amount ofabout 3% to about 10% by weight of the water therein and an organiccrosslinker comprised of polyethylene imine present in the compositionin an amount of about 0.5% to about 4% by weight of water therein. Forexample, a preferred composition of this invention can be comprised of acopolymer of acrylamide and t-butyl acrylate present in an amount ofabout 7% by weight of the water therein and an organic crosslinkercomprised of polyethylene imine present in the composition in an amountof about 1% by weight of water therein.

According to an embodiment, the crosslinkable polymer compositionpreferably has a gelation time of at least about 2 hours when tested ata constant shear rate of 10 l/s, a constant pressure of 270 psi, and aconstant temperature of 250° F. (121° C.). Preferably, the crosslinkablepolymer composition has a gelation time of less than 100 hours whentested at a constant shear rate of 10 l/s, a constant pressure of 270psi, and a constant temperature of 250° F. (121° C.).

It is to be understood, of course, that without undo experimentation,further examples and even more preferred compositions may be determinedby the ordinary routineer with ordinary experimentation within the scopeand spirit of the invention as defined herein.

2. PREFERRED METHODS

In general, the methods of this invention for blocking the permeabilityof a portion of a subterranean formation are comprised of the steps ofintroducing a treatment fluid comprising a crosslinkable polymercomposition according to the invention into the portion of thesubterranean formation, and then allowing the crosslinkable polymercomposition to form a crosslinked gel. Forming the crosslinked gel inthe subterranean formation reduces or completely blocks thepermeability, whereby fluid flow through that portion is reduced orterminated.

More particularly, the method for blocking the permeability of a portionof a subterranean formation penetrated by a wellbore, the methodcomprising the steps of: (a) selecting the portion of the subterraneanformation to be treated; (b) selecting estimated treatment conditions,wherein the estimated treatment conditions comprise temperature over atreatment time; (c) forming a treatment fluid that is a crosslinkablepolymer composition comprising: (i) a water-soluble polymer, wherein thewater-soluble polymer comprises a polymer of at least one non-acidicethylenically unsaturated polar monomer; (ii) an organic crosslinkercomprising amine groups, wherein the organic crosslinker is capable ofcrosslinking the water-soluble polymer; (iii) a salt of a weak Bronstedbase and a Bronsted acid; and (iv) water; (d) selecting thewater-soluble polymer, the crosslinker, the salt of a weak Bronsted baseand a Bronsted acid, and the water, and the proportions thereof, suchthat the gelation time of the treatment fluid is at least 1 hour whentested under the estimated treatment conditions; and (e) injecting thetreatment fluid through the wellbore into the portion of thesubterranean formation. Preferably, the step of injecting is underactual treatment conditions that are within the limits of the estimatedtreatment conditions. According to a further embodiment, the methodfurther comprises the step of allowing the treatment fluid to gel priorto producing hydrocarbons from or through the subterranean formation.

The bottomhole temperature of the portion of the subterranean formationto be treated can be equal to or greater than 80° F. (27° C.).Preferably, the bottomhole temperature of the portion of thesubterranean formation to be treated is equal to or less than 400° F.(204° C.), although higher temperatures may be possible for certaincrosslinkable polymer compositions.

More particularly, these treatment fluids are usually made up justbefore use by mixing the water-soluble polymer, the crosslinker, thegelation retarder, and water, and then injecting the aqueous compositioninto the formation. The composition is preferably kept at below 122° F.(50° C.), e.g., below 86° F. (30° C.) before use.

The introduction of these compositions into the subterranean formationmay, if desired, be preceded by a pre-cooling treatment of the portionof the subterranean formation to be treated, e.g., with cold water tostop premature cross-linking, but preferably the injection process isperformed without such a pretreatment.

The aqueous compositions may be injected into a formation via aproducing well or via a secondary injection well (for use with a waterflood or squeeze technique), for example. The aqueous compositions maysimply be injected into the formation, but preferably they are forcedinto it by pumping.

The well may be shut in for about 1 hour to about 70 hours, for example,to allow the gelling to occur, and then production may be restarted.Preferably, the gelation time of the crosslinkable polymer compositiondoes not exceed about 6 hours under the estimated treatment conditions.Any substantial flowback from the zone can be delayed for at least theexpected gelation time under actual downhole conditions after the stepof injecting the well treatment fluid into the zone.

The compositions for use in the methods according to the invention havethe benefit of a low tendency to crosslinking and gelling in thewellbore (i.e., reduced aggregate build-up) but rapid cross-linking atthe high temperatures of the subterranean formation. They are,therefore, less susceptible to process handling problems. According tothe more preferred embodiments, the treatment fluids and methods arewithout the environmental and other problems associated with the use ofmetal crosslinking agents.

3. EXAMPLES

Halliburton Energy Services, Inc. has employed a crosslinkable polymersystem of a copolymer of acrylamide and t-butyl acrylate, where thecrosslinking agent is polyethylene imine. These materials arecommercially available from Halliburton Energy Services, Inc. as part ofthe H₂Zero™ conformance control service. The H₂Zero™ service employs acombination of HZ10™ polymer and HZ20™ crosslinker. HZ10™ polymer is alow molecular weight polymer consisting of polyacrylamide and anacrylate ester. More particularly, HZ10™ polymer is a co-polymer ofacrylamide and t-butyl acrylate (“PAtBA”). The HZ20™ crosslinker is apolyethyleneimine (which is not chelated). The H₂Zero™ service forconformance control includes mixing the HZ-10™ polymer with the HZ20™crosslinker and injecting the fluid mixture into a well. Unwanted waterintrusion treatment or seal off in oil or gas producing wells can beaddressed by placing permanent sealing systems like H₂Zero™ into thereservoir. The deeper placement of the sealing polymers is the key pointto assure short and long-term success of the water control process.

In high temperature environments, the deeper placement of the sealingHZ10™ polymer of H₂Zero™ service has only been possible using acarbonate salt based retarder system like sodium carbonate bufferingagent, which has a high buffered pH for a 1% solution of about pH 10 toabout 10.5. Lab testing of the H₂Zero™ system using sodium carbonatebuffering agent with the field water, in some cases depending on thewater source and its hardness, has showed salt precipitation problem dueto the carbonate incompatibility or high pH of the final polymersolution. Lowering the pH tends to undesirably shorten the gelationtime.

A salt of a weak Bronsted base and a Bronsted acid in replacement of thecarbonate based salts has been tested. Lab testing showed that a salt ofa weak Bronsted base and a Bronsted acid that is not carbonic acid basedworks also as a retarder system for the H₂Zero™ service.

Lab testing looking for additional types of salts that are not carbonicacid based, as gelation retarders has showed that a salts of a weakBronsted bases and a Bronsted acid other than carbonic acid could workas an effective gelation retarder for the H₂Zero™ system at hightemperatures.

The retardation effect on gelation time of salts of a weak Bronsted baseand a Bronsted acid is evident from the following experimentation. Acrosslinkable polymer composition was used consisting of 350 gal/MgalHZ-10 and 30 gal/Mgal HZ-20 in 2% KCl solution. The composition wastested with either no gel retarder as a control or with different saltsof weak Bronsted bases and acids under conditions of constanttemperature of 190° F. (88° C.), ambient pressure, and static conditions(no shear). Table 1 summarizes the testing results, where “NA” indicatesnot applicable:

TABLE 1 Gelation Time EXAMPLE Gelation Retarder gal/Mgal lb/Mgal (hrs) @190° F. 1 None NA Not 7 applicable 2 Triethanolamine 52 NA 31hydrochloride 3 Ammonium tartrate NA 100 20 4 Ammonium tartrate NA270 >41 5 Ammonium sulfate NA  75 23

The results show that salts of weak Bronsted bases, for exampletriethanolamine and ammonia with either strong Bronsted acids, forexample hydrochloric acid (Example 2) or sulfuric acid (Example 5), orweak Bronsted acids, for example tartaric acid *Examples 3 and 4),extend the cross-linking time when compared to similar compositionswithout using such salts (Example 1). The results also show the desiredcross-linking time can be realized by adjusting the amount of the saltconcentration as shown in Examples 3 and 4. A salt of a weak Bronstedbase and a Bronsted acid as retarder for the crosslinkable polymercompositions comprising amine containing organic crosslinkers isbelieved to be a new approach for avoiding problems associated withcarbonate or formate salt based retarders.

For comparison, the gelation times for an H₂Zero system at such a hightemperature without any retarder is about 7 hours.

Salt of a weak Bronsted base and an acid solutions have exhibited theability to delay the cross-linking for an H₂Zero™ system, which wouldotherwise proceed much more quickly under such conditions. In general,it is believed that a salt of a weak Bronsted base and a Bronsted acidin a concentration of at least 10 lb/Mgal would begin to be effective todelay the gelation time of the H₂Zero™ system. It is expected that theseexamples of salt of a weak Bronsted base and a Bronsted acid as agelation retarder for a crosslinkable polymer composition can beextrapolated to be useful with any water-soluble polymer, wherein thewater-soluble polymer comprises a polymer of at least one non-acidicethylenically unsaturated polar monomer. Such retarders can be usedwithout any problems associated with precipitate formation when used inhard water, sea water or brines which are incompatible with carbonate orformate salts.

4. EXAMPLES ARE ILLUSTRATIVE OF INVENTION

Therefore, the present invention is well adapted to attain the ends andadvantages mentioned as well as those that are inherent therein. Theparticular embodiments disclosed herein are illustrative only, as thepresent invention may be modified and practiced in different butequivalent manners apparent to those skilled in the art having thebenefit of the teachings herein. Furthermore, no limitations areintended to the details of construction or design herein shown, otherthan as described in the claims below. It is, therefore, evident thatthe particular illustrative embodiments disclosed above may be alteredor modified and all such variations are considered within the scope andspirit of the present invention.

While compositions and methods are described in terms of “comprising,”“containing,” or “including” various components or steps, thecompositions and methods also can “consist essentially of” or “consistof” the various components and steps. Whenever a numerical range with alower limit and an upper limit is disclosed, any number and any includedrange falling within the range is specifically disclosed. In particular,every range of values (of the form, “from about a to about b,” or,equivalently, “from approximately a to b,” or, equivalently, “fromapproximately a to b”) disclosed herein is to be understood to set forthevery number and range encompassed within the broader range of values.

Also, the terms in the claims have their plain, ordinary meaning unlessotherwise explicitly and clearly defined by the patentee. Moreover, theindefinite articles “a” or “an”, as used in the claims, are definedherein to mean one or more than one of the element that it introduces.If there is any conflict in the usages of a word or term in thisspecification and one or more patent(s) or other documents that may beincorporated herein by reference, the definitions that are consistentwith this specification should be adopted.

What is claimed is:
 1. A treatment fluid for use in a subterraneanformation, the treatment fluid comprising: (a) a water-soluble polymer,wherein the water-soluble polymer comprises a polymer of at least onenon-acidic ethylenically unsaturated polar monomer; (b) a polymericorganic cross linker comprising amine groups, wherein the polymericorganic crosslinker is capable of crosslinking the water-solublepolymer; (c) a salt of a weak Bronsted base and a Bronsted acid, whereinthe salt of a weak Bronsted base and a Bronsted acid is not a formate ora carbonate and is present in a concentration of at least about 0.1% byweight of water (10 lb/Mgal); and (d) water; wherein the treatment fluidcomprises a crosslinkable polymer composition.
 2. The treatment fluidaccording to claim 1, wherein the non-acidic ethylenically unsaturatedpolar monomer is acrylamide.
 3. The treatment fluid according to claim1, wherein the water-soluble polymer comprises: (i) at least onenon-acidic ethylenically unsaturated polar monomer, and (ii) at leastone polymerizable ethylenically unsaturated ester.
 4. The treatmentfluid according to claim 3, wherein the polymerizable ethylenicallyunsaturated ester is t-butyl ester.
 5. The treatment fluid according toclaim 1, wherein the water-soluble polymer is poly(acrylamide/t-butylacrylate).
 6. The treatment fluid according to claim 1, wherein thewater-soluble polymer is soluble in water to an extent of at least 10g/l when measured in a sodium chloride solution of 32 g/l of sodiumchloride in deionized water at 25° C.
 7. The treatment fluid accordingto claim 1, wherein the polymeric organic crosslinker is apolyalkyleneimine.
 8. The treatment fluid according to claim 1, whereinthe polymeric organic crosslinker is polyethyleneimine.
 9. The treatmentfluid according to claim 1, wherein the salt of a weak Bronsted base anda Bronsted acid is selected from the group consisting of ammoniumtartrate, ammonium citrate, ammonium dihydrogen phosphate, ammoniummonohydrogen phosphate, ammonium phosphate, triethanolaminehydrochloride, anilinium hydrochloride, trisopropanolaminehydrochloride, pyridinium hydrochloride, and any combination thereof.10. The treatment fluid according to claim 1, wherein the salt of a weakBronsted base and a Bronsted acid is not an ammonium halide.
 11. Thetreatment fluid according to claim 1, wherein the salt of a weakBronsted base and a Bronsted acid is present in a concentration of atleast 25 lb/Mgal of the water.
 12. The treatment fluid according toclaim 1, wherein the polymeric organic crosslinker is selected from thegroup consisting of a polyvinylamine, a poly(vinylamine-co-vinylalcohol,and any combination thereof.
 13. The treatment fluid according to claim1, wherein the crosslinkable polymer composition has a gelation time ofat least about 2 hours when tested at a constant shear rate of 10 l/s, aconstant pressure of 270 psi, and a constant temperature of 250° F.(121° C.).
 14. A treatment fluid for use in a subterranean formation,wherein the treatment fluid comprises: (a) a water-soluble polymercomprising a copolymer of: (i) at least one non-acidic ethylenicallyunsaturated polar monomer, and (ii) at least one polymerizableethylenically unsaturated ester; (b) a polyethyleneimine capable ofcross-linking the water-soluble polymer; (c) a salt of a weak Bronstedbase and a Bronsted acid, wherein the salt of a weak Bronsted base and aBronsted acid is not a formate or a carbonate and is present in aconcentration of at least about 0.1% by weight of water (10 lb/Mgal);and (d) water; wherein the treatment fluid is a crosslinkable polymersolution.
 15. The treatment fluid according to claim 14, wherein thecrosslinkable polymer composition has a gelation time of at least about2 hours when tested at a constant shear rate of 10 l/s, a constantpressure of 270 psi, and a constant temperature of 250° F. (121° C.).16. A method for blocking the permeability of a portion of asubterranean formation penetrated by a wellbore, the method comprisingthe steps of: (a) selecting the portion of the subterranean formation tobe treated; (b) selecting estimated treatment conditions, wherein theestimated treatment conditions comprise temperature over a treatmenttime; (c) forming a treatment fluid that is a crosslinkable polymercomposition comprising: (i) a water-soluble polymer, wherein thewater-soluble polymer comprises a polymer of at least one non-acidicethylenically unsaturated polar monomer; (ii) a polymeric organiccrosslinker comprising amine groups, wherein the polymeric organiccrosslinker is capable of crosslinking the water-soluble polymer; (iii)a salt of a weak Bronsted base and a Bronsted acid, wherein the salt ofa weak Bronsted base and a Bronsted acid is not a formate or a carbonateand is present in a concentration of at least about 0.1% by weight ofwater (10 lb/Mgal); and (iv) water; (d) selecting the water-solublepolymer, the crosslinker, the salt of a weak Bronsted base and aBronsted acid, and the water, and the proportions thereof, such that thegelation time of the treatment fluid is at least 1 hour when testedunder the estimated treatment conditions; and (e) injecting thetreatment fluid through the wellbore into the portion of thesubterranean formation.
 17. The method according to claim 16, whereinthe salt of a weak Bronsted base and a Bronsted acid is selected fromthe group consisting of ammonium tartrate, ammonium citrate, ammoniumdihydrogen phosphate, ammonium monohydrogen phosphate, ammoniumphosphate, triethanolamine hydrochloride, anilinium hydrochloride,trisopropanolamine hydrochloride, pyridinium hydrochloride, and anycombination thereof.
 18. The method according to claim 16, wherein thesalt of a weak Bronsted base and a Bronsted acid is not an ammoniumhalide.
 19. The method according to claim 16, wherein the step ofinjecting is under actual treatment conditions that are within thelimits of the estimated treatment conditions.
 20. The method accordingto claim 16, further comprising the step of: allowing the treatmentfluid to gel in the formation.
 21. The method according to claim 20,further comprising, after the step of allowing, the step of producinghydrocarbons from or through the subterranean formation.
 22. A methodfor blocking the permeability of a portion of a subterranean formationpenetrated by a wellbore, the method comprising the steps of: (a)forming a treatment fluid that is a crosslinkable polymer compositioncomprising: (i) a water-soluble polymer, wherein the water-solublepolymer comprises a polymer of at least one non-acidic ethylenicallyunsaturated polar monomer; (ii) a polymeric organic crosslinkercomprising amine groups, wherein the polymeric organic crosslinker iscapable of crosslinking the water-soluble polymer; (iii) a salt of aweak Bronsted base and a Bronsted acid, wherein the salt of a weakBronsted base and a Bronsted acid is not a formate or a carbonate and ispresent in a concentration of at least about 0.1% by weight of water (10lb/Mgal); and (iv) water; (b) introducing the treatment fluid throughthe wellbore into the portion of the subterranean formation; and (c)allowing the crosslinkable polymer composition to form a crosslinked gelin the subterranean formation.
 23. The method according to claim 22,wherein the salt of a weak Bronsted base and a Bronsted acid is selectedfrom the group consisting of ammonium tartrate, ammonium citrate,ammonium dihydrogen phosphate, ammonium monohydrogen phosphate, ammoniumphosphate, triethanolamine hydrochloride, anilinium hydrochloride,trisopropanolamine hydrochloride, pyridinium hydrochloride, and anycombination thereof.
 24. The method according to claim 22, wherein thesalt of a weak Bronsted base and a Bronsted acid is not an ammoniumhalide.
 25. The method according to claim 22, further comprising thestep of: (d) selecting the water-soluble polymer, the crosslinker, thesalt of a weak Bronsted base and a Bronsted acid, and the water, and theproportions thereof, such that the gelation time of the treatment fluidis at least 1 hour at a bottom hole temperature of the portion of thesubterranean formation.
 26. The method according to claim 22, furthercomprising, after the step of allowing, the step of producinghydrocarbons from or through the subterranean formation.
 27. The methodaccording to claim 22, wherein the crosslinkable polymer composition hasa gelation time of at least about 2 hours when tested at a constantshear rate of 10 l/s, a constant pressure of 270 psi, and a constanttemperature of 250° F. (121° C.).